Niacin is one of the recommended drugs of first choice for the treatment of primary hypercholesterolemia whereas lovastatin is a recently released hypocholesterolemic agent which acts as a specific inhibitor of cholesterol biosynthesis. The mechanism by which niacin lowers plasma lipoprotein concentrations is not clear and this study examines the effect of this drug on cholesterol biosynthesis assessed by the 24 hour urinary excretion of mevalonic acid.